Distributed traveling-wave mach-zehnder modulator driver

ABSTRACT

A distributed traveling-wave Mach-Zehnder modulator driver having a plurality of modulation stages that operate cooperatively (in-phase) to provide a signal suitable for use in a 100 Gb/s optical fiber transmitter at power levels that are compatible with conventional semiconductor devices and conventional semiconductor processing is described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of U.S. patentapplication Ser. No. 14/618,989, filed Feb. 10, 2015, which claimspriority to U.S. Provisional Application No. 61/937,683, filed Feb. 10,2014, each of which is hereby incorporated by reference herein in itsentirety.

FIELD OF THE INVENTION

The invention relates to optical transmitters in general, andparticularly to an optical driver for a Gigabit/second transmitter.

BACKGROUND OF THE INVENTION

Optical interconnects offer promising solutions to data transmissionbottlenecks in supercomputers and in data-centers as well as otherapplications. Adopting higher channel data rates can greatly reduce thecomplexity in optical communication systems and/or further improveinterconnect capacity and density.

The most important requirement on the driver amplifier is the outputvoltage swing. The state-of-the-art driver amplifier in CMOS/BiCMOS canoutput 3 Vpp at 40 Gb/s, consuming 1.35 W DC power. See for example, C.Knochenhauer, J. Scheytt, and F. Ellinger, “A Compact, Low-Power40-GBit/s Modulator Driver With 6-V Differential Output Swing in 0.25 umSiGe BiCMOS,” Solid-State Circuits, IEEE Journal of, vol. 46, no. 5, pp.1137-1146, 2011.

At higher data rates it is difficult to maintain or improve theavailable drive voltage without substantial advances in the fabricationprocess. This trend is at odds with the increasingly higher drivevoltage required by modulators at higher speed.

There is a need for improved drivers for use in optical data handlingsystems.

SUMMARY OF THE INVENTION

According to one aspect, the invention features a distributed travelingwave modulator. The distributed traveling wave modulator comprises adifferential optical input forreceiving an optical input carrier signaland a differential optical output for providing a modulated opticalcarrier signal; a plurality N of optical phase-shifters connected inseries connection as N sequential modulators between the differentialoptical input and the differential optical output, where N is an integerequal to or greater than 2; a plurality N of driver amplifier stages,each having a respective differential driver amplifier input and adifferential driver amplifier output; N-1 delay/relay stages, eachhaving a respective differential delay/relay input and a differentialdelay/relay output; a first of the plurality N of driver amplifierstages having its input connected to a differential electrical datainput; each of the first N-1 of the plurality N of driver amplifierstages having its output connected to a respective input of a successiveone of the N-1 delay/relay stages; each of the N-1 delay/relay stageshaving its respective differential delay/relay output connected to thedifferential driver amplifier input of a successive one of the last N-1of the plurality N of driver amplifier stages; and each of the pluralityN of driver amplifier stages having a differential signal outputconnected to a respective one of the N sequential modulators; whereineach driver amplifier stage includes only a single type of transistor toenable high-speed operation.

According to another aspect, the invention relates to a method ofmodulating an optical signal, comprising the steps of: receiving theoptical signal to be modulated at an optical input port; applying aplurality N of sequential optical phase shifts to the optical signal byoperation of a plurality N of fixed-length optical phase-shiftersconnected in series connection as N sequential modulators, where N isgreater than or equal to 2, each of the N-1 phase shifts after the firstof the N phase shifts delayed by a time calculated to apply each of theN-1 phase shifts after the first of the N phase shifts at a respectivetime when the optical signal passes a respective one of the N-1sequential modulators after the first modulator, and providing amodulated optical signal at an optical output port.

The foregoing and other objects, aspects, features, and advantages ofthe invention will become more apparent from the following descriptionand from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention can be better understood withreference to the drawings described below, and the claims. The drawingsare not necessarily to scale, emphasis instead generally being placedupon illustrating the principles of the invention. In the drawings, likenumerals are used to indicate like parts throughout the various views.

FIG. 1A is a block diagram of a typical optical transmitter and receiverof the prior art.

FIG. 1B is a graph that illustrates the power scaling versus data ratefor prior art silicon traveling-wave modulators.

FIG. 2A is a circuit block diagram of a distributed traveling-waveMach-Zehnder (TWMZ) modulator driver that operates according toprinciples of the invention.

FIG. 2B is a schematic circuit diagram of a driver amplifier stage ofthe distributed traveling-wave Mach-Zehnder modulator driver of FIG. 2A.

FIG. 2C is a schematic circuit diagram of a delay/relay stage of thedistributed traveling-wave Mach-Zehnder modulator driver of FIG. 2A.

FIG. 2D is an image of a chip that embodies the distributedtraveling-wave Mach-Zehnder modulator driver of FIG. 2A. The chip has awidth of 1 mm and a length of 2.9 mm. DC bias is provided by thestructure at the right side of the chip.

FIG. 3A is a graph that illustrates the results of a distributed TWMZdriver post-layout simulation showing 100 Gb/s eye-diagrams at thedriver outputs.

FIG. 3B is a graph that illustrates the results of a distributed TWMZdriver post-layout simulation showing 100 Gb/s eye-diagrams after the SiTWMZ. The differential output is 2 Vpp at 25 Ω impedance; data isshifted by 13 picoseconds (ps) between each output stage.

DETAILED DESCRIPTION

As illustrated in FIG. 1A, the analog front-end in a typical prior artoptical transceiver includes a modulator driver and a transimpedanceamplifier that serve as interfaces between a high-speed optical channeland lower speed digital electronics. Data is provided to the MUX and ismodulated onto a laser carrier in an optical fiber using the driver andthe modulator. The modulated carrier travels down the fiber. At areceiver, a photodetector samples the optical carrier and atransimpedance amplifier and DEMUX provide an electrical output signalthat represents the data provided to the MUX.

FIG. 1B shows the expected voltage requirement vs. data rate. As shownin FIG. 1B, the required drive voltage for the prior art modulator at100 Gb/s is >6 Vpp (on each swing-end output), which is far from apractical voltage in existing CMOS/BiCMOS technology.

We describe systems and methods to provide ultra-high channel rate (40to 100 Gb/s) optical transmitters in silicon-based electronics andphotonics technology.

We have described another design of a traveling wave modulator in RanDing, Yang Liu, Qi Li, Yisu Yang, Yangjin Ma, Kishore Padmaraju, AndyEu-Jin Lim, Guo-Qiang Lo, Keren Bergman, Tom Baehr-Jones, and MichaelHochberg, “Design and characterization of a 30-GHz bandwidth low-powersilicon traveling-wave modulator,” Optics Communications (availableonline Feb. 7, 2014).

In various embodiments of the present invention, the followingassumptions are made: Cpn is 230 fF/mm, Rpn is 5.5 Ω-mm, Vπ Lπ is 2.0V-cm, device bandwidth is 70% data rate, a differential-drive geometryis used, and an equivalent of Vit/3 swing generate acceptable opticalmodulation amplitude. As an example, we describe a distributed TWMZdriver that can be fabricated in a 130-nm SiGe BiCMOS process in orderto bridge the gap between the increasingly higher drive-voltage requiredby modulators and limited available driver output voltage swing fromelectronics at higher data rates.

FIG. 2A is a circuit block diagram of a distributed traveling-waveMach-Zehnder (TWMZ) modulator driver that operates according toprinciples of the invention. An optical input waveguide 220 receives anoptical signal that is to be modulated, and splits the signal in two,one portion of the signal passing through wave shifters 210, 212, 214and 216, and the other portion of the signal passing through waveshifters 210′, 212′, 214′ and 216′. In a preferred embodiment, theoptical signal is split into two portions having equal intensities. Inone embodiment, the wave shifter pairs (210, 210′), (212, 212′), (214,214′), and (216, 216′), are Mach-Zehnder interferometers with fixedoptical lengths to minimize power consumption and increase speed. Theoptical signals are recombined and exit the modulator at optical port240. In some embodiments, the drive circuitry which will now bedescribed is attached to the chip using flip-chip bump bonding,illustrated by bonding interface 230.

As shown in the circuit block diagram in FIG. 2A, the driver amplifiertakes 400 mVpp input signals at each of the differential inputs In+ andIn−, and delays and amplifies the signals to four pairs of differentialoutputs with 13 ps delay between each output stage. Each output swings 1Vpp single-ended (2 Vpp differential) on a 25 Ω impedance. The output isintentionally configured to be open-collector to offer the flexibilityto drive both 25 Ω and 50 Ω impedance TWMZ sections (without and withnear-end termination, respectively). The termination resistors are notillustrated in the schematic shown in FIG. 2B. They could be introducedduring the packaging step or they could be monolithically integrated onthe modulator side. The open-collector nature of the proposed driverenables the TWMZ sections to be designed to have different impedance,increasing the size of the optimization space and the number of possibleconfigurations.

In the preferred embodiment of FIG. 2A, there are illustrated aplurality of N of optical phase shifter pairs, where N=4. In otherembodiments, one can use a different number N of optical phase shifterpairs, so long as N is greater than or equal to 2. In the embodimentshown, the distributed traveling-wave Mach-Zehnder (TWMZ) modulatordriver has four driver amplifier stages 250 (illustrated in greaterdetail in FIG. 2B) and three delay/relay stages 260 (illustrated ingreater detail in FIG.2C).

In other embodiments, one can use other kinds of optical phase shiftersin place of the TWMZ, so long as the number of optical phase shifters isgreater than or equal to 2.

FIG. 2B and FIG. 2C are circuit diagrams that illustrate preferredembodiments of the driver amplifier stages 250 and the delay/relaystages 260 of FIG. 2A, respectively. The driver stage 250, shown in FIG.2B, starts with a pair of emitter followers 252 a and 252 b equippedwith termination resistors for efficient coupling to the data source orto the previous stage of the driver. The received signal is thenamplified using a differential pair 254 a and 254 b, and subsequentlybuffered again using emitter followers 256 a and 256 b. Finally, thesignal is split and applied to two open collector cascode output drivers258 a and 258 b, one driving the TWMZ segment, and one 259 a and 259 bamplifying the signal for the following stage of the driver. Eachmodulator includes a driver amplifier stage 250, which includes only asingle type of transistor to enable high-speed operation. The preferredtype is NPN bipolar transistor; however other possible transistorsinclude a PNP, a MOSFET (NFET or PFET, either one) or even a HEMT or apHEMT.

In one embodiment, the integration interface between silicon TWMZsections and the driver circuits is expected to be flip-chipbump-bonding. A 40 fF parasitic capacitance is assumed for each signalconnection. The optical delay of each TWMZ section plus opticalwaveguide wiring matches the delay between the amplifier stages so thatthe modulations constructively add. As an additional step to improve theperformance, we have incorporated pre-amplification in the driver outputto extend the length of TWMZ sections that can be driven at 100 Gb/s byabout 40%. The driver pre-amplifier stage 254 a and 254 b is shown inFIG. 2B immediately after the input emitter follower stage 252 a and 252b. Without the pre-amplifier 254 a and 254 b the overall driver 250would not be able to have the gain-bandwidth product required to achievethe target 100 Gb/s data rate, especially when driving a longer TWMZsection.

The example circuit described above consumes 1.5 W power overall. The DCbias structures illustrated on the right of the chip (FIG. 2D) controlthe on and off states of each main driver stage individually, which is auseful feature for testing before integration. The bias voltages Vtb andVtb main of the driver section 250 shown in FIG. 2B are generatedseparately in the DC Bias structures shown in FIG. 2D. Each of thedriver sections 250 can then be enabled or disabled by turning thecorresponding bias voltages on and off. This increases the flexibilityof the driver 250, allowing integration with different types of TWMZs.For example, if the TWMZ has only three sections, the fourth section ofthe driver (the one providing outputs Out4+ and Out4− in FIG. 2D) can beshut down, reducing the overall power dissipation. The feature can alsobe useful in other scenarios. For example, if optical modulationresulting from the action of fewer stages of the driver is found to beadequate, the redundant stages can be shut down. In addition, slightvariation in the biasing voltages of individual stages can be used tooptimize delays, gains and the overall performance of the driver-TWMZsystem.

In the embodiment shown in FIG. 2D the chip or substrate is silicon. Inother embodiments, the substrate can be fabricated from a semiconductor,which may be different from a silicon or silicon-on-insulator wafer.

Post-layout simulations at 100 Gb/s is shown in FIG. 3A and FIG. 3B. TheTWMZ sections are modeled using an equivalent circuit model.Bump-bonding parasitics are taken into account. As one can see, similarelectrical eye quality is maintained in each stage output and this isachieved by scaling the transmission lines and device sizes in eachstage. The eye-diagrams at the end of the TWMZ (FIG. 3B) provide aconservative estimation of the optical eye-diagrams.

In the driving scheme used in the embodiment of FIG. 2A through FIG. 2D,the overall drive voltage requirement is linearly lowered byaccumulating modulation from four sections of TWMZ of 750 μm length,achieving an overall modulator length of 3 mm, which is similar to a 40Gb/s device illustrated in FIG. 1B. The present device provides apractical solution for a 100 Gb/s optical transmitter.

In operation, an optical wave (or an optical signal) to be modulated isexpected to be received at an input port such as 220, subjected to asuccession of N modulations performed by successive ones of a pluralityN a plurality N of optical phase-shifters connected in series connectionas N sequential modulators, where N is greater than or equal to 2, eachof the N-1 phase shifts after the first of the N phase shifts delayed bya time calculated to apply each of the N-1 phase shifts after the firstof the N phase shifts at a respective time when the optical signalpasses a respective one of the N-1 sequential modulators after the firstmodulator, and providing a modulated optical signal at an optical outputport, such as port 240.

The apparatus described above can be used for performing such opticalmodulation as just described.

Definitions

Unless otherwise explicitly recited herein, any reference to anelectronic signal or an electromagnetic signal (or their equivalents) isto be understood as referring to a non-volatile electronic signal or anon-volatile electromagnetic signal.

Theoretical Discussion

Although the theoretical description given herein is thought to becorrect, the operation of the devices described and claimed herein doesnot depend upon the accuracy or validity of the theoretical description.That is, later theoretical developments that may explain the observedresults on a basis different from the theory presented herein will notdetract from the inventions described herein.

Any patent, patent application, patent application publication, journalarticle, book, published paper, or other publicly available materialidentified in the specification is hereby incorporated by referenceherein in its entirety. Any material, or portion thereof, that is saidto be incorporated by reference herein, but which conflicts withexisting definitions, statements, or other disclosure materialexplicitly set forth herein is only incorporated to the extent that noconflict arises between that incorporated material and the presentdisclosure material. In the event of a conflict, the conflict is to beresolved in favor of the present disclosure as the preferred disclosure.

While the present invention has been particularly shown and describedwith reference to the preferred mode as illustrated in the drawing, itwill be understood by one skilled in the art that various changes indetail may be affected therein without departing from the spirit andscope of the invention as defined by the claims.

1-19. (canceled)
 20. A driver amplifier for a distributed traveling-wavemodulator comprising a plurality of driver amplifier stages, each driveramplifier stage comprising: a differential driver amplifier input forreceiving a differential input signal from a data source or a previousstage; a first pair of emitter followers including terminationresistors; a pre-amplifier comprising a differential pair; a buffercomprising a second pair of emitter followers; a splitter for splittingthe input signal into first and second differential output signals; adifferential driver modulator driver output for receiving the firstdifferential output signal for driving a modulator segment; and adifferential driver signal output for amplifying the second differentialoutput signal for providing the amplified second differential output toa subsequent driver amplifier stage.
 21. The driver amplifier accordingto claim 20, wherein each driver amplifier stage includes only a singletype of transistor to enable high-speed operation.
 22. The driveramplifier according to claim 20, further comprising a plurality of DCbias elements for independently varying biasing voltages of individualdriver amplifier stages.
 23. The driver amplifier according to claim 22,wherein each DC bias element is capable of controlling an on state andan off state of a respective driver amplifier stage for shutting downredundant driver amplifier stages.
 24. The driver amplifier according toclaim 22, further comprising a delay/relay stage between driveramplifier stages for matching an optical delay between modulatorsegments, whereby modulations constructively add.
 25. The driveramplifier according to claim 24, wherein a plurality of the DC biaselements are configured to individually control said plurality ofdelay/relay stages; wherein each DC bias element is capable ofcontrolling an on state and an off state of a respective delay/relaystage for shutting down redundant delay/relay stages.
 26. The driveramplifier according to claim 20, further comprising a delay/relay stagebetween driver amplifier stages for matching an optical delay betweenmodulator segments, whereby modulations constructively add.
 27. Thedriver amplifier according to claim 26, further comprising a pluralityof DC bias elements, each DC bias element configured to individuallycontrol one of said plurality of delay/relay stages; wherein each DCbias element is capable of controlling an on state and an off state of arespective delay/relay stage for shutting down redundant delay/relaystages.
 28. The driver amplifier according to claim 20, wherein eachoutput of each driver amplifier stage is configured to be anopen-collector driver for driving modulator segments with differentimpedances.
 29. The driver amplifier according to claim 20, wherein eachoutput of each driver amplifier stage is configured to be anopen-collector cascode driver for driving modulator segments withdifferent impedances.
 30. The driver amplifier according to claim 20,wherein each output of each driver amplifier stage is configured to bean open-collector driver for driving both 25 Ω and 50 Ω impedancemodulator segments.
 31. The driver amplifier according to claim 20,wherein the plurality of driver amplifier stages comprises four.
 32. Thedriver amplifier according to claim 26, wherein the plurality of driveramplifier stages comprises four, and the number of delay/relay stagescomprises three.